Fluidized bed combustor having integral solids separator

ABSTRACT

A circulating fluidized bed furnace (10) is disclosed wherein a non-cyclonic particulate separator (18) is integrally disposed in the flue gas flow path between the furnace gas outlet (14) and the flue gas duct (16). The separator (18) comprises an arcuate duct (20 ) having a curvilinear inner wall (24), the portion thereof which is disposed across the inlet to the flue gas duct (16) having a plurality of openings (28) therein which provide a flow area through which a portion of the flue gas pass into the flue gas duct (16). Gas/solids separation is accomplished as the flue gas turns sharply from its arcuate path to pass through the openings (28) in the inner wall (24). The momentum of the particulate solids and the centrifugal forces acting thereon prevent the solids from sharply turning and cause the solids to continue on their arcuate flow path through the duct (20) in the remaining flue gas to a solids collection means (40) opening to the solids outlet (26) of the arcuate duct (20).

BACKGROUND OF THE INVENTION

The present invention relates to fluidized bed combustors wherein aparticulate fuel is combusted in a fluidized state, and, moreparticularly, to such a combustor having an integral solids separatorfor removing particulate material carried over in the flue gas generatedin the combustion chamber.

In a typical present day fluidized bed combustor, particulate fuel, suchas coal having a top size ranging up to about 6.5 millimeters, istypically fed to and combusted within a furnace chamber in a fluidizedstate in a fluidizing gas at relatively low temperatures ranging fromabout 760° C. to 925° C. Fluidized bed furnaces are particularlyadaptable to burning sulfur containing fuels as the particulate materialfluidized within the furnace may include a sulfur absorbent, mostcommonly crushed limestone, in addition to the particulate fuel.Fluidizing air, which also serves as combustion air, is supplied to thefurnace from an air plenum located beneath the combustion chamber. In atypical circulating fluidized bed, also referred to as a fast fluidizedbed, the velocity or flow rate of the fluidizing air being passedupwardly into the furnace is maintained at a level sufficiently high toentrain most of the particulate material present within the furnace suchthat a substantial portion of the particulate material within thecombustion chamber is carried therefrom with the flue gas.

Accordingly, it is necessary on circulating or fast fluidized bedcombustors to provide a means for removing the particulate materialcarried over from the combustion chamber with the flue gas prior toventing the flue gas to the atmosphere. As this particulate materialtypically contains a significant amount of unburned fuel, it is furtheradvantageous to collect the particulate material from the gas stream forrecycle to the combustion chamber of the furnace for further combustionin order to increase the efficiency of fuel utilization. Additionally,the particulate material removed from the gas stream will generallycontain a significant amount of unreacted sulfur absorbent which is alsorecirculated to the combustion chamber of the furnace in order toincrease the efficiency of utilization of the sulfur absorbent.

The most common means for separating the particulate solids from theflue gas passing from the combustion chamber prior to venting same tothe atmosphere is a cyclone separator. A circulating fluidized bedsystem utilizing a cyclone for a particulate collection is illustratedin U.S. Pat. No. 4,111,158. As shown, the cyclone is typically installedimmediately downstream of the furnace in order to facilitate recycle ofthe collected particulate material to the furnace. As the particulatesolids entrained in the flue gas will have a temperature reflecting thetemperature of the flue gas, the cyclone separator will be exposed toboth hot solids and hot gases and therefore must be designed towithstand temperatures ranging as high as 1600° C. under normaloperation. Additionally, the cyclone separator must be designed tosurvive operation in a highly erosive environment as the particulateloading in the flue gas will result in a significant impact of erosiveparticles on the side walls of the cyclone. Accordingly, the type ofcyclone suitable for utilization in a circulating fluidized bed systemhas a high capital cost and frequently a high operating cost associatedtherewith.

It would be desirable to substitute a less expensive separating meansfor the cyclone on a typical circulating fluidized bed furnace. Examplesof such circulating fluidized bed furnace systems utilizing non-cyclonicseparators are illustrated in U.S. Pat. Nos. 4,442,797 and 4,538,549which show, respectively, the use of momentum separators and the use ofimpact separators for removing particulate material from the flue gaspassing from the combustion chamber.

In U.S. Pat. Nos. 4,442,797, the flue gas passing from the furnacechamber to the flue gas vent duct must exit the furnace chamber througha plurality of vertical slot-like openings formed by bending thewaterwall tubes in the upper wall of the combustion chamber inwardlyfrom the plane of the wall to provide a flow area between adjacenttubes. A plenum chamber extends in belt-like fashion around the upperregion of the furnace chamber to receive the flue gas which passesthrough these tubes. As the flue gas passes through the slot-likeopenings formed between tubes, the flue gas drops in velocity below theentrainment level and the particles carried therein drop out of the fluegas stream and are directed to a collection hopper for recycle to thefurnace chamber.

In U.S. Pat. No. 4,538,549, impact beams are provided at and immediatelydownstream of the gas outlet of the furnace chamber. The flue gasleaving the combustion chamber of the furnace passes between the rows ofimpact beams while the particulate solids contained therein, due totheir flow inertia, strike the impact beams. As a consequence ofstriking the impact beams, the particles lose their momentum and dropout of the gas stream into a collection hopper for recycle to thefurnace while the flue gas passes on to the vent stack.

It is an object of the present invention to provide a fluidized bedcombustor having an integral non-cyclonic solids separator for removingthe hot particulate solids carried in a flue gas stream prior to ventingthe flue gas stream to the atmosphere.

SUMMARY OF THE INVENTION

A circulating fluidized bed combustor system for burning a particulatefuel in a fluidized state in a fluidizing gas comprising a furnaceenclosure defining a combustion chamber having a hot gas outlet, a fluegas duct disposed downstream of the furnace enclosure for venting theflue gas generated in the combustion chamber, separator means integrallyincorporated into the gas flow path downstream of the furnace chamberand upstream of the flue gas duct for separating particulate solidscarried over from the furnace chamber in the flue gas, and solidscollection means for receiving particulate solids from the separatormeans.

The separator means comprises an arcuate duct having an inlet opening tothe gas outlet of the furnace enclosure for receiving the hot flue gasfrom the chamber and an outlet spaced from the inlet. The arcuate ductcomprises spaced curvilinear inner and outer walls extending from theinlet of the duct to the outlet of the duct, and a pair of spacedsidewalls extending transversely between the spaced inner and outerwalls. A portion of the curvilinear inner wall of the arcuate ductcomprising separator means is disposed across the flue gas duct inletand has a plurality of openings formed therein for providing a flow areathrough which a first major portion of the hot flue gas passes from thearcuate duct of the separator means into the flue gas duct for ventingto the atmosphere. A second minor portion of the flue gas passes throughthe outlet of the arcuate duct into the solid collection means andcarries with it the particulate solids separated from the first portionof the flue gas passing through the inner wall of the arcuate duct ofthe separator means. Due to the curvilinear shape of the arcuate duct,centrifugal forces are generated which act on the particles passingthrough the separator means so as to cause the particles within the fluegas passing therethrough to concentrate along the curvilinear outer wallof the arcuate duct.

The particulate solids contained in the first portion of the flue gaspassing through the openings in the inner wall of the arcuate duct havesufficient momentum so that they do not follow the path of the firstportion of the flue gas through the openings in the inner wall of thearcuate duct, but rather pass to the outlet of the arcuate duct and arecarried in a second minor portion of the flue gas along the outercurvilinear wall of the arcuate duct through the outlet into the solidscollection means.

Preferably, the portion of the inner curvilinear wall of the arcuateduct of the separator means disposed across the inlet to the flue gasduct comprises a plurality of spaced louvers disposed transverselybetween the spaced sidewalls of the arcuate duct so as to provide aplurality of openings therebetween. Each of the louvers is disposed atan acute angle with the local tangent to the curvilinear inner wallwhereby the first portion of the flue gas which passes through theopenings therein must sharply change its direction of flow, while thesolid particles, being under the influence of much higher inertiaforces, separate from the first portion of flue gas as it changesdirection of flow to pass through the openings between the louvers, andcontinue along the curvilinear flow path through the arcuate duct to beconcentrated by centrifugal forces in a second minor portion of the fluegas which passes along the outer curvilinear wall of the arcuate duct tothe solid collection means disposed at the outlet of the arcuate duct.

BRIEF DESCRIPTION OF THE DRAWING

The features and advantages and objects of the present invention will beevident from the following description of the illustrated embodimentsthereof in the accompanying drawing wherein:

FIG. 1 is a side elevational view illustrating a fluidized bed combustorincorporating an integral non-cyclonic solids separator in accordancewith the present invention;

FIG. 2 is a side elevational view of an alternate embodiment of afluidized bed boiler system incorporating a non-cyclonic solidsseparator in accordance with the present invention;

FIG. 3 is a side elevational view of an alternate embodiment of afluidized bed boiler system incorporating a non-cyclonic solidsseparator in accordance with the present invention;

FIG. 4 is a cross-sectional plan view taken along line 4--4 of FIG. 1;

FIG. 5 is a sectional elevation view taken along line 5--5 of FIG. 1;

FIG. 6 is a enlarged sectional elevational view showing a curvilinearfloor portion of the solid separator wherein the floor portion is formedof a plurality of louvers; and

FIG. 7 is a cross-sectional view taken along line 7--7 of FIG. 6.

DESCRIPTION OF A PREFERRED EMBODIMENT

Referring now to the drawings, there is depicted therein a fluidized bedfurnace 10 wherein a sulfur containing fuel, such as particulate coal,is combusted in a fluidized state with additional particulate materialwhich includes a sulfur oxide absorbent. The particulate fuel iscombusted in the fluidizing air within the combustion chamber 12 definedby the furnace enclosure 10 to generate a hot flue gas which exits thecombustion chamber 12 through the furnace gas outlet 14. The hot fluegas leaving the combustion chamber 12 through the furnace gas outlet 14is passed to the flue gas vent duct 16 for venting to the atmosphere.Typically, convection surface, not shown, is disposed in the flue gasvent duct 16 to cool the gas prior to venting the flue gas to theatmosphere through a stack, not shown.

As the particulate fuel in a circulating or fast fluid bed is combustedin a fluidized state in a fluidizing gas having a velocity sufficient tonot only fluidize but also entrain a substantial portion of theparticulate material present within the combustion chamber 12, theparticulate material entrained in the flue gas leaving the combustionchamber 12 through the gas outlet 14 must be removed from the flue gasstream prior to venting the flue gas stream to the atmosphere. Intypical prior art circulating fluidized bed furnace systems, a cycloneseparator was typically disposed downstream of the furnace gas outlet 14intermediate the combustion chamber and the flue gas vent duct to removea major portion of the particulate material for recycle to the furnacechamber. The flue gas passing through the flue gas vent duct 16 wouldstill contain some particulate material which would be typically removedfrom the flue gas stream by passing the flue gas stream through a fabricfilter type collector prior to venting the flue gas through the stack.As noted previously, it would be advantageous to eliminate the cycloneseparator in favor of a much simpler and lower capital cost separationmeans.

In accordance with the present invention, the means 18 for separatingthe particulate solids carried over from the combustion chamber in theflue gas passing through the furnace gas outlet 14 comprises an integralpart of the furnace system in the form of an arcuate duct 20 having aninlet opening to the gas outlet 14 of the furnace enclosure 10 forreceiving the hot flue gas from the combustion chamber 12. The arcuateduct 20 is formed of a curvilinear outer wall 22, a curvilinear innerwall 24, and a pair of spaced sidewalls extending transverselytherebetween. It is to be understood that "inner wall" refers to thecurvilinear wall of the arcuate duct having the lesser radius ofcurvature, while "outer wall" refers to the curvilinear wall of thearcuate duct having the greater radius of curvature. In the embodimentshown in FIG. 1, the furnace gas outlet 14 of the furnace 10 is in avertical plane and the arcuate duct 20 of the separator means of thepresent invention comprises a downwardly curved duct subtending an angleof 90 degrees between a vertically disposed inlet mating with thefurnace gas outlet 14 and a horizontally disposed solids outlet 26. Inthe embodiment shown in FIG. 2, the furnace gas outlet 14 is in ahorizontal plane and the arcuate duct 20 of the separator means of thepresent invention comprises a semi-torodial duct subtending an angle of180 degrees between a horizontally disposed inlet mating with thefurnace gas outlet 14 and a horizontally disposed solids outlet 26spaced therefrom at the opposite end of the duct. In the embodimentshown in FIG. 3, the furnace gas outlet 14 is in a horizontal plane andthe arcuate duct 20 of the separator means of the present inventioncomprises a downwardly curved duct subtending an angle of 90 degreesbetween a horizontally disposed inlet mating with the furnace gas outlet14 and a vertically disposed solids outlet 26.

A portion of the curvilinear inner wall 24 of the arcuate duct 20 isdisposed across the inlet to the flue gas duct 16 and has a plurality ofopenings 28 therein which provide a flow area through which a firstportion of the hot flue gases pass from the arcuate duct 20 of theseparator means to the flue gas duct 16. Gas-solids separation isaccomplished in the separator means of the present invention as the fluegas turns sharply from its arcuate path through the duct 20 to passthrough the openings 28 in the curvilinear inner wall 24 of the duct.Due to the centrifugal forces exerted on the particulate solids in theflue gas as it passes through the arcuate duct 20, the particulatesolids have sufficient momentum that they continue along their arcuatepath through the duct 20 and are unable to follow the flue gas as itturns sharply from its arcuate flow path to pass through the holes 28 inthe curvilinear inner wall of the duct 20. The flue gas having passedthrough the curvilinear inner wall 24 of the arcuate duct 20 enters theflue gas vent duct 16 at a significantly reduced solids content. Thisreduced solids content flue gas continues through the flue gas vent duct18 over convective cooling surface disposed therein and is, ifnecessary, passed therefrom to a fabric filter or multiclone mechanicalseparator for further removal of particulate material prior to ventingto the atmosphere through a stack, not shown.

The substantial portion of solid particulate material in the hot fluegas passing from the furnace chamber through the furnace gas outlet 14passes through the solids outlet 26 of the arcuate duct 20 into solidscollection means 40 which opens to the solids outlet 26 of the arcuateduct 20 at the end of the arcuate duct opposite from the inlet thereto.These solids are concentrated and entrained in a second minor portion ofthe flue gas passing through the arcuate duct 20 and are carried throughthe solid collection means 40 and recycled back to the combustionchamber 12 within the furnace enclosure 10. It is necessary andadvantageous to recycle this particulate material as it will containunburned particulate fuel and also unutilized particulate sulfurabsorbent.

Solids separation is also enhanced in accordance with Applicants'invention in that the separator means is in the form of an arcuate ducthaving a curvilinear outer wall. As a result of this duct configuration,the solids within the flue gas passing from the furnace gas outlet 14are drawn by centrifugal forces to concentrate in the flue gas passingalong the outer wall of the arcuate duct 14 and away from the portion ofthe flue gas which will pass along and through the inner wall of thearcuate duct 20.

As best seen in FIGS. 4 and 5, the solids outlet 26 of the arcuate duct20 opens at the base of the curvilinear outer wall 22 to the solidscollection means 40 which preferably comprises a receiving hopper 42 anda plurality of downcomer conduits 44 which extend downwardly from thereceiving hopper 42 and thence interconnect with the furnace enclosure10 to open to the combustion chamber 12 thereby providing a flow paththrough which the particulate solids entrained in the second minorportion of the flue gas passing through the arcuate duct 20 arereinjected into the furnace chamber. As the particulate solids and fluegas passing through the downcomer conduits 44 will be at a hightemperature since they have not traversed any cooling surface in thatthey have bypassed the flue gas vent duct 16, it is desirable to provideinternal or external cooling means operatively associated with one ormore of the downcomer conduits 44, such as the internally disposedcooling coil 46 illustrated in FIG. 3, so as to cool the solids prior toreinjection into the furnace chamber and take advantage of the heatcontent contained therein to heat a liquid or a vapor for usefulpurposes.

In the preferred embodiment of the present invention, the portion of thecurvilinear inner wall 24 of the arcuate duct 20 disposed across theinlet to the flue gas duct 16 is formed of a plurality of spaced louvervanes 30 disposed transversely between the spaced sidewalls of thearcuate duct 20 so as to provide a plurality of openings therebetween.The louvers 30 are disposed so as to extend at an acute angle to thelocal tangent to the curvilinear inner wall so that the flue gas passingthrough the arcuate duct along a path substantially parallel to thecurvilinear inner wall 24 of the arcuate duct 20 must turn sharply backupon itself to pass through the openings between adjacent louver vanes30. As mentioned previously, the solid particles in the flue gas are,due to their momentum, unable to traverse the sharp turn traversed bythe flue gas passing through the curvilinear inner wall and instead passonwardly through the arcuate duct 20 to the solids outlet 26 thereof.

It is also preferred that the cross-sectional area of the arcuate duct20 continuously decrease in flow area in the direction of flue gas flowat least through that portion of the duct bounded by the perforatedportion of the curvilinear inner wall 24 disposed atop the flue gas duct16. If the cross-sectional area of the duct 20 remained constant, thevelocity of the flue gas passing therethrough would decrease as a firstportion of the flue gas passes through the perforated floor in the fluegas duct. As a consequence, the momentum of the particulate solids inconcentrating in second portion of the flue gas would decrease as thevelocity of the flue gas decreases resulting in the potential forpremature precipitation of the particulate solids within the arcuateduct 20. This result is avoided by continuously decreasing thecross-sectional area of the arcuate duct 20 as the first portion of theflue gas vents through the perforated inner wall 24 into the flue gasduct 16 such that the velocity of the second portion of the flue gasremains high enough to maintain the particulate solids therein in anentrained state so as to carry the solids into the solids collectionmeans 40.

Although the louver vanes 30 may take many forms of construction, in thepresently preferred embodiment, as best seen in FIGS. 6 and 7, theindividual louver vanes 30 are mounted to curvilinear fluid cooled tubes32 at an acute angle with the curvilinear tube 32 to form the portion ofthe curvilinear inner wall 24 which is disposed atop the flue gas duct16. Further, as shown in FIG. 6, each louver vane 30 may be formed of aplurality of segments 34 disposed in side by side relationship acrossthe inner curvature of the arcuate duct 20 with each individual segmentmounted to a fluid cooled tube 32 as opposed to being a single integrallouver vane extending across the inner wall 24 of the duct 20. Thelouver vanes 30 may be made of ceramic or metal as appropriate for eachgiven installation.

A particular advantage of the louver vane wall design of Applicants'invention is that the louver vanes 30 are disposed at a fairly smallarcuate angle with the gas flow such that the surface of the vanes willnot be exposed to significant erosive impact from particulate solids inthe flue gas passing through the curvilinear duct. Impact typeseparators used in the prior art are necessarily exposed to high erosionas the particles themselves impinge directly upon the separator in orderto slow their momentum so that they will separate from the gas stream.Also, in prior art cyclonic type separators, the whirling flow ofparticulate solids within the separator creates high erosion of thewalls of the cyclone separator. The separator means of the presentinvention is not subject to the highly erosive effects of the particlesas the particles do not directly impact upon the surface but rather flowalong a curvilinear path through the arcuate duct of Applicants'separator. Applicants' separator depends upon the centrifugal momentumof the particles generated when the flue gas passes from the furnaceoutlet through the arcuate duct and upon the perforated curvilinearinner wall requires the flue gas to sharply turn in order to passthrough the openings in the inner wall which results in the solids toinherently separate from the flue gas.

We claim:
 1. A fluidized bed combustor system for combusting aparticulate fuel comprising:a. a furnace enclosure defining a combustionchamber for burning a particulate fuel in a fluidized state in afluidizing gas to generate a hot flue gas and having a gas outlet abovesaid bed for passing the hot flue gas from the combustion chamber; b. aflue gas duct disposed downstream of the furnace enclosure having aninlet for receiving the flue gas generated in the combustion chamber; c.separator means for separating particulate solids carried over from thecombustion chamber in the flue gas, said separator means comprising anarcuate duct having an inlet opening to the gas outlet of the furnaceenclosure for receiving the hot flue gas from the combustion chamber andan outlet spaced from said inlet and interconnected to said inlet by acurvilinear inner wall, a curvilinear outer wall, and a pair of spacedsidewalls extending therebetween, a portion of the curvilinear innerwall being disposed across the inlet to flue gas duct, said portionhaving a plurality of openings therein providing a flow area throughwhich a first portion of the hot gas passes from the arcuate duct of theseparator means through the inlet to the flue gas duct; and d. solidscollection means opening to the outlet of the arcuate duct of theseparator means for receiving a second portion of the flue gas togetherwith the particulate solids separated from the first portion of the fluegas passing through the inner wall of the arcuate duct of the separatormeans.
 2. A fluidized bed combustor system as recited in claim 1 whereinthe curvilinear inner wall of the arcuate duct of the separator meansdisposed across the inlet to the flue gas duct comprises a plurality ofspaced louvers disposed transversely between the spaced sidewalls of thearcuate duct so as to provide a plurality of openings therebetween, eachof said louvers disposed at an acute angle with a tangent to thecurvilinear inner wall at its location whereby the flue gas passingthrough the openings between the plurality of louvers changes itsdirection of flow while the particulate solids carried in the flue gasseparate therefrom and continue traveling an arcuate flow path throughthe separator means.
 3. A fluidized bed combustor system as recited inclaim 2 wherein the arcuate duct of the separator means has across-sectional area which continuously decreases in flow area in thedirection of flue gas flow therethrough.
 4. A fluidized bed combustorsystem as recited in claim 1 wherein the arcuate duct of the separatormeans comprises a downwardly curved duct subtending an angle of 90degrees between a vertically disposed inlet and a horizontally disposedoutlet.
 5. A fluidized bed combustor system as recited in claim 1wherein the arcuate duct of the separator means comprises a curvilinearduct subtending an angle of 180 degrees between a horizontally disposedinlet and a horizontally disposed outlet.
 6. A fluidized bed combustorsystem as recited in claim 1 wherein the solids collection meanscomprises at least one downcomer conduit connected to the outlet of thearcuate duct for receiving the second portion of the flue gas passingthrough the outlet of the arcuate duct together with the particulatesolids carried therein, the downcomer conduit providing a flow paththrough which the second portion of the flue gas and the particulatesolids carried therein are conveyed for discharge into the combustionchamber of the furnace enclosure.
 7. A fluidized bed combustor system asrecited in claim 6 further comprising a conduit cooling means associatedwith at least one downcomer conduit for cooling the particulate solidspassing therethrough.